How to validate email address with telnet

When collecting email addresses you will always validate it someway often the validation only consists of validating the format of the email address.

Bu what if you wan’t to know if an email address really exits without sending an actual email and ask (which is the absolute best way ofc)

Then  you can use the command line with nslookup and telnet.

First thing we need to know what the domains mx-record is if there is any.

As you can see has two mx-records if the domain of the email you are trying to validate don’t have any mx-records you can stop right here because that domain can’t accept any emails.

If you find a mx you can go on with telnet like this:

If there is a mailserver on the other side you will get a response code 220.

Then you can start sending SMTP-commands:

The first command must be HELO or EHLO the difference between the two is that HELO does not respond with any information about the server which EHLO does.

Then MAIL FROM: and you should get a 250 OK

Then RCPT TO: if you get a 250 OK here there is a pretty good guess that the email you have is correct and exists. This is not 100% proof of the email existing for example yahoo will always answer 250 OK and only after the DATA command will they tell you if the email address is ok or not. And you do not want to go there because if it exists the recipient will get your test email.

The reason why we test another recipient with RCPT TO: is to see if the mail server has catch-all configured. If you get 250 OK on your completely made up address there is a good chance that the server has catch-all and the it’s no way to know if the original email exists either.

The last command is just QUIT.

validate email with telnet

Validating email addresses with telnet


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